In modern aircraft power systems, as well as in other power systems, use is sometimes made of high-energy permanent magnet material; e.g., samarium-cobalt, in generators and motors. In the generator application, the permanent-magnet rotor exhibits no losses and it is an ideally-simple generator configuration. In addition, the three-phase stator-winding can be easily fluid-cooled.
The problems, however, with the above permanent magnet machine are that materials such as samarium cobalt are scarce, expensive, heavy, and present fabrication difficulties since the material is available primarily in small sections. A further and more serious problem is that the potential energy of the machine can self-destruct (electrically) if a stator short occurs. This is because the generator excitation, the permanent magnet field, cannot be shut off, other than by mechanically-isolating the drive-source from the generator.
On the other hand, squirrel-cage rotor induction generators are extremely rugged and reliable. Such generators use relatively inexpensive materials in the rotor and they can normally be easily fabricated. There are, however, also certain problems associated with induction generators. First, to operate as a generator, the rotor must be driven above its synchronous speed; i.e., in a negative-slip condition. Second, the machine cannot operate as a generator unless there is at least one synchronous generator available in the system to excite it. Finally, the machine cannot supply either its own excitation or (lagging) reactive kilovars.
It is a primary object then, of the present invention, to provide a power generation system, particularly for aircraft, that utilizes an extremely reliable induction machine as a power generator within the system.
It is a further object of the present invention, to develop a multi-machine power system which utilizes at least one synchronous machine in conjunction with an induction machine, all in a single housing.
These and other objects of the present invention will become more readily apparent after a full consideration of the following description of the instant invention and the several advantages thereof enumerated herein.
A variety of electric motors which utilize multiple rotors within a single motor housing are known in the prior art. In U.S. Pat. No. 3,743,873 to de Jong, for example, a synchronous electric machine, including two induction rotors supported by a single shaft, is disclosed. The arrangement of rotors is utilized to increase the useful flux and improve the synchronous electric machine.
In another type of machine, use of both permanent magnet rotors and induction rotors about a single shaft of a synchronous electric motor is disclosed. See U.S. Pat. Nos. 2,949,553 to Benoit, 3,209,185 to Draper, 3,521,097 to Trinter, and 3,973,137 to Drubina. All of these latter patents, to some degree or another, operate via induction start of a synchronous motor design.
In another prior art machine, disclosed in U.S. Pat. No. 907,176 to Richardson, two motors are mechanically coupled in parallel planes within a common magnetic frame. Such a dual motor arrangement is designed for heavy work where the motor must be started under heavy load.
Finally, U.S. Pat. No. 285,548 to Ball shows an arrangement wherein an "integral" exciter is utilized to excite a dynamo-electric machine. The arrangement provides, via a basic machine design, an efficient electric generator for incandescent lighting without the expense of a separate exciter.
All of the aforementioned prior art patents are directed to power systems quite different from the permanent-magnet-rotor excited induction-generator system of the present invention, and in fact, do not address themselves to problems relating to utilization and excitation of induction generators.
A slightly-more complex system which utilizes a synchronous generator as an exciter for an induction generator is disclosed in co-pending U.S. patent application, Ser. No. 220,865, for "A Differential Drive for Frequency Control of Induction Generators", assigned to the assignee herein. The system, hereby incorporated by reference herein, includes the capability of adjusting the slip frequency as a function of the load and the input speed. The instant invention is a somewhat simpler and more straight-forward system which utilizes a permanent magnet generator as an exciter for an induction generator wherein fixed ratio drives are utilized with the input power. While the system does not include the benefit of being able to adjust the slip frequency as a function of load and input speed, it does include other benefits. These benefits include, but are not limited to, a somewhat less complex drive arrangement, lower cost of drives, somewhat less complex starter systems when it is desired to use the induction generator as a motor starter, as might be the case in aircraft installations. The latter benefit is attributable to the fact that the induction generator is directly connected to the engine.